Thrust is produced in a monopropellant thruster, or reaction engine, in the following stages: (1) A monopropellant fluid (liquid or gaseous) that is usually pressurized is injected onto a catalyst bed. (2) When the monopropellant comes in contact with the catalyst it decomposes, or ignites. Decomposition of the monopropellant may occur in one reaction or in multiple sequential reactions. Then (3) the decomposition products are exhausted through the exit cone or nozzle to create thrust. The thrust, or specific impulse, is dependant on many variable including engine design, size, and energy produced by propellant decomposition.
A typical monopropellant catalyst consists of discrete active metal particles dispersed on a ceramic carrier, or substrate. The active metal particles catalyze, or reduce the activation energy for, monopropellant decomposition upon contact. The purpose of the catalyst substrate is to (i) increase the surface area of active metal to provide more sites for propellant decomposition and (ii) stabilize the active metal particles, i.e., prevent them from migrating or sintering, which leads to particle growth and/or loss of active metal surface area. During thruster operation, the substrate is exposed to the propellant and intermediate species that form during propellant decomposition as well as decomposition products that are extremely corrosive, particularly at elevated temperatures (>1000° C.). This leads to degradation of the substrate, followed by catalyst deactivation. A long life monopropellant catalyst must be composed of a substrate that is resistant to corrosion from the propellant and its decomposition products and intermediate species and resistant to degradation under thruster operating conditions.
Reduced toxicity high-energy-density ionic salt monopropellants, including but not limited to monopropellants containing an oxidizer such as hydroxylammonium nitrate (HAN, [HO—NH3+]NO3−) and one or more fuels in highly concentrated solutions containing water, ethanol or a suitable solvent or without a solvent are described as replacements for hydrazine-based propellants. The new monopropellants, which will hereinafter sometimes be referred to as ionic salt monopropellants or high-energy-density ionic salt monopropellants and which include HAN-based ionic salt monopropellants, offer lower toxicity, lower flammability, lower vapor pressure, lower freezing-point temperature, and higher density-specific impulse than hydrazine-based monopropellants.
Liquid monopropellants, including but not limited to HAN-based ionic salt monopropellants, can be decomposed by passing them over a solid catalyst bed. The catalyst decreases the activation energy required for monopropellant decomposition, thus allowing for combustion at lower temperatures than required for pure thermal decomposition. However, the decomposition and combustion reactions degrade the catalyst. As a result, a typical monopropellant thruster can only be fired for a limited number of pulses, or until the catalyst fails due to loss of catalytic mass or loss of catalyst activity as described above.
The high-adiabatic-decomposition-temperatures of the described HAN-based ionic salt monopropellants render conventional catalysts ineffective when applied to these formulations. The adiabatic flame temperature of the HAN-based ionic salt monopropellants exceeds 1800° C., whereas hydrazine possesses an adiabatic flame temperature of only 900° C. In addition, decomposition of the HAN-based ionic salt monopropellants produces highly oxidizing species such as oxygen (O2), acidic species such as HNO3, and water vapor that are highly corrosive to metals as well as ceramics such as alumina (Al2O3) that are typically used in conventional catalysts.
Use of high-energy-density ionic salt monopropellants, including ionic salt monopropellants, as a replacement for the current state-of-art hydrazine monopropellant can potentially increase thruster performances. However, use of hot burning high-energy-density ionic monopropellants requires use of catalysts, chamber materials, and bed plates in the thruster/reaction engine that can survive in the monopropellant decomposition environment at temperatures exceeding 1600° C. and as high as 2000° C.
Conventional, prior art catalysts such as Ir/Al2O3, Pt/Al2O3, LCH-210, LCH-207, LCH-227, Shell 405 or S-405 that were developed for use with hydrazine cannot withstand the higher operating temperatures and the more corrosive environment encountered in decomposing high-energy-density HAN-based ionic salt monopropellants.
ZrO2 is an amphoteric refractory oxide and has demonstrated a good acid resistance in HAN thruster environments. However, it suffers from a destructive tetragonal-to-monoclinic phase transformation due to a volumetric change of 3%-5% or more associated with this phase transformation. Repeated heating and cooling cycles, such as those encountered in a rocket engine, would result in a complete loss of the mechanical integrity of ZrO2.
Stabilizers, typically MgO, CaO, Y2O3, La2O3 or CeO2, are added in necessary concentrations to fully stabilize ZrO2 in the cubic phase or to partially stabilize ZrO2 in tetragonal phase to provide transformation toughening and prevent the spontaneous tetragonal-to-monoclinic phase transformation and destruction of the material that would otherwise occur upon heating and cooling over a given temperature range as described above. However, these stabilizers are all basic and are susceptible to acid-base reactions in acidic environments such as those encountered in rocket engines operating with ionic salt-based monopropellants. Partially stabilized (t-ZrO2) or fully stabilized (c-ZrO2) goes through an aging process where the basic stabilizers come out of solid solution and subsequently react with the acidic species in the HAN thruster environment. This acid-base reaction effectively removes the basic stabilizer from the zirconia and thus increases the rate of precipitation of the stabilizers from ZrO2 and accelerates the aging and destabilization of ZrO2. The sequence of precipitation and removal by acid-base reaction prevents the basic stabilizer, once precipitated, from going back into solid solution with ZrO2 upon heating and re-stabilizing the material. In addition, some of the conventional stabilizers mentioned here, such as MgO and CaO, are extremely hydrophilic and thus after precipitation from ZrO2 may potentially be removed from the material in the presence of water vapor or steam that can exist in the thruster environment.
Problems observed during rocket engine tests containing conventional catalysts with new monopropellants include excessive sintering of catalyst, void formation, increase in pressure drop, fracturing of catalyst granules, fine formation, fragmentation of the catalyst granules due to thermal shock, leaching of the catalyst by acids, and rapid loss of catalyst activity. Catalysts such as LCH-237 and Sienna's SSC-0103 that consist of an Ir coated ZrO2-based carrier containing traditional stabilizers such as CeO2, Y2O3, or CaO can provide over 30 minutes of lifetime but some missions require longer lifetimes. The lifetime of these catalysts are limited by the aging of the stabilized ZrO2 carriers, and leaching of the stabilizers by acids or steam during service that leads to fracturing of catalyst granules, fine formation, fragmentation of the catalyst granules due to thermal shock, and rapid loss of catalyst activity.
Ceramic materials that have been evaluated as catalyst carriers for use with HAN-based ionic salt monopropellants include transition metal oxides such as Al2O3, TiO2, ZrO2, CeO2—ZrO2, Y2O3—ZrO2 (Kirchnerova, J., Klvana, D. (2000) “Design Criteria for High Temperature Combustion Catalysts,” Catalysis Lett, Vol. 67, p. 175), refractory carbides and nitrides such as SiC and Si3N4 (Rodrigues, J. A. J et al., (1997), “Nitride and Carbide of Molybdenum and Tungsten as Substitutes of Iridium for the Catalyst Used for Space Communication”, Catalysis Lett., Vol. 45, P. 1-3), transition metal-based and alkaline earth-based perovskites (Savrun, E. and Schmidt, E. W., (2001), “High Temperature Catalyst for Nontoxic Monopropellant”, Air Force Research Laboratories SBIR Phase I Final Report, AFRL-PR-ED-TR-2001-0012; Savrun, E. et al., “Novel Catalysts for HAN/HEHN Based Monopropellants”, NASA Glenn Research Center SBIR Phase I final Report, NAS3-02025) and transition metal substituted lanthanum-strontium hexaaluminates (Tejuca, L. G., Fierro, J. L. G., and Tascon, J. M. D., (1989) “Structure and Reactivity of Perovskite-Type Oxides”, Adv. Catalysis, Vol. 36, P. 237). The combined stabilizing effects and corrosion resistance of In2O3 in ZrO2 are cited in U.S. Pat. No. 5,288,205 to Jones, R., 1994, “India-stabilized Zirconia Coating for Composites,” and in the following reference: Jones, R. L. and Mess, D. 1992, “India as a Hot Corrosion-Resistant Stabilizer for Zirconia,” Journal of American Ceramics Society, volume 75, pages 1818-1821. The combined stabilizing effects and corrosion resistance of Sc2O3 in ZrO2 are cited in Jones, R. L., 1989 “Scandia-stabilized Zirconia for Resistance to Molten Vanadate-sulfate Corrosion,” Surface and Coatings Technology, volume 39/40, pages 89-96. The combined stabilizing effects and corrosion resistance of SnO2 in ZrO2 are cited in U.S. Pat. No. 5,312,585 to Jones, R. L., 1994, “Corrosion Inhibition in High Temperature Environment”. The combined stabilizing effects of Ga2O3 in ZrO2 and its resistance to water or steam are cited in U.S. Pat. No. 5,279,995 to Hiroaki Tanaka et al., 1994, “Zirconia Ceramics.”